To create the mailing list, first run a report for the members of the campaign and the desired fields that you wish to put on the mailing label in Salesforce. Then export this report to excel .CSV.
For details on how to obtain the Report please read: How do I get a list of Campaign Members?
Once you have that please open up Microsoft Word and go to : Tools | Letters and Mailings | Show Mail Merge Toolbar
Choose Main Document Setup (it's the first button on the Mail Merge Toolbar). Choose Labels. It will then ask you to choose your label options (it depends on the kind of label you're printing on).
You will now see your Word doc in a format that is broken up into the labels.
Click Open Data Source (the button to the right of Main Document Setup). Now locate your report in the .CSV format.
Now you have to choose the Insert Merge Field button (sixth from the left on the Mail Merge Toolbar) and select the appropriate fields that you wish to merge your data to.
After you have chosen all the fields, make sure that you have clicked the Propagate Labels button (11th from the left on the Mail Merge Toolbar). This will put in "<<Next Record>>" on all the ensuing labels and allow your mail merge to function properly.
Now you are ready to merge the data from the .CSV to the word doc. To do so, click on the View Merge Data button which looks like <<ABC>> and is 8th from the left on the Mail Merge Toolbar.
Now you should see the first page of your successfully created mailing labels. To view the rest of the labels, you can scroll through the pages by using the "Next Record" button which is 14th from the left, or insert a number as to which page you would like to view. To view all labels at once, simply choose Merge to New Document which is 4th from the right.
The mail merge button will not work to create mailing address labels for campaign members. The reason is that none of the data fields necessary for a mail merge document are stored under the Campaign object.